What is slot in shader editor
The slots are for giving different faces different material, all in the same object. If you have only one slot, you can use the button you marked to give your object material.

In Edit Mode you can select faces and assing them to certain slots. Lets say you want two slots. You press the little down arrow next to Slot 1 and press + to add a new slot. Select a face, select slot 2 and press assign. Now you can give the selected face different material by pressing the button you marked.